1 =============================
2 NO-MMU MEMORY MAPPING SUPPORT
3 =============================
45The kernel has limited support for memory mapping under no-MMU conditions, such
6as are used in uClinux environments. From the userspace point of view, memory
7mapping is made use of in conjunction with the mmap() system call, the shmat()
8call and the execve() system call. From the kernel's point of view, execve()
9mapping is actually performed by the binfmt drivers, which call back into the
10mmap() routines to do the actual work.
1112Memory mapping behaviour also involves the way fork(), vfork(), clone() and
13ptrace() work. Under uClinux there is no fork(), and clone() must be supplied
14the CLONE_VM flag.
1516The behaviour is similar between the MMU and no-MMU cases, but not identical;
17and it's also much more restricted in the latter case:
1819 (*) Anonymous mapping, MAP_PRIVATE
2021 In the MMU case: VM regions backed by arbitrary pages; copy-on-write
22 across fork.
2324 In the no-MMU case: VM regions backed by arbitrary contiguous runs of
25 pages.
2627 (*) Anonymous mapping, MAP_SHARED
2829 These behave very much like private mappings, except that they're
30 shared across fork() or clone() without CLONE_VM in the MMU case. Since
31 the no-MMU case doesn't support these, behaviour is identical to
32 MAP_PRIVATE there.
3334 (*) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, !PROT_WRITE
3536 In the MMU case: VM regions backed by pages read from file; changes to
37 the underlying file are reflected in the mapping; copied across fork.
3839 In the no-MMU case:
4041 - If one exists, the kernel will re-use an existing mapping to the
42 same segment of the same file if that has compatible permissions,
43 even if this was created by another process.
4445 - If possible, the file mapping will be directly on the backing device
46 if the backing device has the NOMMU_MAP_DIRECT capability and
47 appropriate mapping protection capabilities. Ramfs, romfs, cramfs
48 and mtd might all permit this.
4950 - If the backing device device can't or won't permit direct sharing,
51 but does have the NOMMU_MAP_COPY capability, then a copy of the
52 appropriate bit of the file will be read into a contiguous bit of
53 memory and any extraneous space beyond the EOF will be cleared
5455 - Writes to the file do not affect the mapping; writes to the mapping
56 are visible in other processes (no MMU protection), but should not
57 happen.
5859 (*) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, PROT_WRITE
6061 In the MMU case: like the non-PROT_WRITE case, except that the pages in
62 question get copied before the write actually happens. From that point
63 on writes to the file underneath that page no longer get reflected into
64 the mapping's backing pages. The page is then backed by swap instead.
6566 In the no-MMU case: works much like the non-PROT_WRITE case, except
67 that a copy is always taken and never shared.
6869 (*) Regular file / blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
7071 In the MMU case: VM regions backed by pages read from file; changes to
72 pages written back to file; writes to file reflected into pages backing
73 mapping; shared across fork.
7475 In the no-MMU case: not supported.
7677 (*) Memory backed regular file, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
7879 In the MMU case: As for ordinary regular files.
8081 In the no-MMU case: The filesystem providing the memory-backed file
82 (such as ramfs or tmpfs) may choose to honour an open, truncate, mmap
83 sequence by providing a contiguous sequence of pages to map. In that
84 case, a shared-writable memory mapping will be possible. It will work
85 as for the MMU case. If the filesystem does not provide any such
86 support, then the mapping request will be denied.
8788 (*) Memory backed blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
8990 In the MMU case: As for ordinary regular files.
9192 In the no-MMU case: As for memory backed regular files, but the
93 blockdev must be able to provide a contiguous run of pages without
94 truncate being called. The ramdisk driver could do this if it allocated
95 all its memory as a contiguous array upfront.
9697 (*) Memory backed chardev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
9899 In the MMU case: As for ordinary regular files.
100101 In the no-MMU case: The character device driver may choose to honour
102 the mmap() by providing direct access to the underlying device if it
103 provides memory or quasi-memory that can be accessed directly. Examples
104 of such are frame buffers and flash devices. If the driver does not
105 provide any such support, then the mapping request will be denied.
106107108============================
109FURTHER NOTES ON NO-MMU MMAP
110============================
111112 (*) A request for a private mapping of a file may return a buffer that is not
113 page-aligned. This is because XIP may take place, and the data may not be
114 paged aligned in the backing store.
115116 (*) A request for an anonymous mapping will always be page aligned. If
117 possible the size of the request should be a power of two otherwise some
118 of the space may be wasted as the kernel must allocate a power-of-2
119 granule but will only discard the excess if appropriately configured as
120 this has an effect on fragmentation.
121122 (*) The memory allocated by a request for an anonymous mapping will normally
123 be cleared by the kernel before being returned in accordance with the
124 Linux man pages (ver 2.22 or later).
125126 In the MMU case this can be achieved with reasonable performance as
127 regions are backed by virtual pages, with the contents only being mapped
128 to cleared physical pages when a write happens on that specific page
129 (prior to which, the pages are effectively mapped to the global zero page
130 from which reads can take place). This spreads out the time it takes to
131 initialize the contents of a page - depending on the write-usage of the
132 mapping.
133134 In the no-MMU case, however, anonymous mappings are backed by physical
135 pages, and the entire map is cleared at allocation time. This can cause
136 significant delays during a userspace malloc() as the C library does an
137 anonymous mapping and the kernel then does a memset for the entire map.
138139 However, for memory that isn't required to be precleared - such as that
140 returned by malloc() - mmap() can take a MAP_UNINITIALIZED flag to
141 indicate to the kernel that it shouldn't bother clearing the memory before
142 returning it. Note that CONFIG_MMAP_ALLOW_UNINITIALIZED must be enabled
143 to permit this, otherwise the flag will be ignored.
144145 uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this
146 to allocate the brk and stack region.
147148 (*) A list of all the private copy and anonymous mappings on the system is
149 visible through /proc/maps in no-MMU mode.
150151 (*) A list of all the mappings in use by a process is visible through
152 /proc/<pid>/maps in no-MMU mode.
153154 (*) Supplying MAP_FIXED or a requesting a particular mapping address will
155 result in an error.
156157 (*) Files mapped privately usually have to have a read method provided by the
158 driver or filesystem so that the contents can be read into the memory
159 allocated if mmap() chooses not to map the backing device directly. An
160 error will result if they don't. This is most likely to be encountered
161 with character device files, pipes, fifos and sockets.
162163164==========================
165INTERPROCESS SHARED MEMORY
166==========================
167168Both SYSV IPC SHM shared memory and POSIX shared memory is supported in NOMMU
169mode. The former through the usual mechanism, the latter through files created
170on ramfs or tmpfs mounts.
171172173=======
174FUTEXES
175=======
176177Futexes are supported in NOMMU mode if the arch supports them. An error will
178be given if an address passed to the futex system call lies outside the
179mappings made by a process or if the mapping in which the address lies does not
180support futexes (such as an I/O chardev mapping).
181182183=============
184NO-MMU MREMAP
185=============
186187The mremap() function is partially supported. It may change the size of a
188mapping, and may move it[*] if MREMAP_MAYMOVE is specified and if the new size
189of the mapping exceeds the size of the slab object currently occupied by the
190memory to which the mapping refers, or if a smaller slab object could be used.
191192MREMAP_FIXED is not supported, though it is ignored if there's no change of
193address and the object does not need to be moved.
194195Shared mappings may not be moved. Shareable mappings may not be moved either,
196even if they are not currently shared.
197198The mremap() function must be given an exact match for base address and size of
199a previously mapped object. It may not be used to create holes in existing
200mappings, move parts of existing mappings or resize parts of mappings. It must
201act on a complete mapping.
202203[*] Not currently supported.
204205206============================================
207PROVIDING SHAREABLE CHARACTER DEVICE SUPPORT
208============================================
209210To provide shareable character device support, a driver must provide a
211file->f_op->get_unmapped_area() operation. The mmap() routines will call this
212to get a proposed address for the mapping. This may return an error if it
213doesn't wish to honour the mapping because it's too long, at a weird offset,
214under some unsupported combination of flags or whatever.
215216The driver should also provide backing device information with capabilities set
217to indicate the permitted types of mapping on such devices. The default is
218assumed to be readable and writable, not executable, and only shareable
219directly (can't be copied).
220221The file->f_op->mmap() operation will be called to actually inaugurate the
222mapping. It can be rejected at that point. Returning the ENOSYS error will
223cause the mapping to be copied instead if NOMMU_MAP_COPY is specified.
224225The vm_ops->close() routine will be invoked when the last mapping on a chardev
226is removed. An existing mapping will be shared, partially or not, if possible
227without notifying the driver.
228229It is permitted also for the file->f_op->get_unmapped_area() operation to
230return -ENOSYS. This will be taken to mean that this operation just doesn't
231want to handle it, despite the fact it's got an operation. For instance, it
232might try directing the call to a secondary driver which turns out not to
233implement it. Such is the case for the framebuffer driver which attempts to
234direct the call to the device-specific driver. Under such circumstances, the
235mapping request will be rejected if NOMMU_MAP_COPY is not specified, and a
236copy mapped otherwise.
237238IMPORTANT NOTE:
239240 Some types of device may present a different appearance to anyone
241 looking at them in certain modes. Flash chips can be like this; for
242 instance if they're in programming or erase mode, you might see the
243 status reflected in the mapping, instead of the data.
244245 In such a case, care must be taken lest userspace see a shared or a
246 private mapping showing such information when the driver is busy
247 controlling the device. Remember especially: private executable
248 mappings may still be mapped directly off the device under some
249 circumstances!
250251252==============================================
253PROVIDING SHAREABLE MEMORY-BACKED FILE SUPPORT
254==============================================
255256Provision of shared mappings on memory backed files is similar to the provision
257of support for shared mapped character devices. The main difference is that the
258filesystem providing the service will probably allocate a contiguous collection
259of pages and permit mappings to be made on that.
260261It is recommended that a truncate operation applied to such a file that
262increases the file size, if that file is empty, be taken as a request to gather
263enough pages to honour a mapping. This is required to support POSIX shared
264memory.
265266Memory backed devices are indicated by the mapping's backing device info having
267the memory_backed flag set.
268269270========================================
271PROVIDING SHAREABLE BLOCK DEVICE SUPPORT
272========================================
273274Provision of shared mappings on block device files is exactly the same as for
275character devices. If there isn't a real device underneath, then the driver
276should allocate sufficient contiguous memory to honour any supported mapping.
277278279=================================
280ADJUSTING PAGE TRIMMING BEHAVIOUR
281=================================
282283NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages
284when performing an allocation. This can have adverse effects on memory
285fragmentation, and as such, is left configurable. The default behaviour is to
286aggressively trim allocations and discard any excess pages back in to the page
287allocator. In order to retain finer-grained control over fragmentation, this
288behaviour can either be disabled completely, or bumped up to a higher page
289watermark where trimming begins.
290291Page trimming behaviour is configurable via the sysctl `vm.nr_trim_pages'.